In many practical cases, a ship, which is a practical embodiment of a vessel, has one or more compartments for containing water, which are partially delimited by the ship's hull, wherein the hull is provided with one or more openings at the position of each of the compartments for allowing exchange of water between the compartments and the ship's environment. Such compartments, which are often referred to as sea chests, may be used for taking in water to be used as ballast water or fire extinguishing water, for example. According to another possibility, assuming that the ship is of the engine-driven type, one or more sea chests may be used for accommodating a so-called box cooler for cooling the fluid of an engine cooling system of the ship, in which case the ship's hull is provided with openings for enabling a continuous flow of water through the sea chests and thereby achieving an effective cooling effect as desired. Usually, a box cooler as may be present in a sea chest comprises a plurality of tubes for containing and transporting the fluid to be cooled in their interior, wherein entry and exit openings are arranged in the hull at the position of the sea chest so that water can enter the sea chest, flow over the tubes in the sea chest, and exit the sea chest through natural flow and/or under the influence of motion of the ship.
In order to allow a sufficient flow of water through an opening of a sea chest on the one hand and to prevent items (including marine animals) which are small enough to pass through the opening from entering the sea chest on the other hand, it is practical for the opening to be equipped with a grating. As is generally known, a grating comprises one or more elements which are suitable for serving as a barrier extending in an opening. Normally, such elements have an elongated shape. In case a grating comprises a plurality of elements, the elements are often positioned in a regularly spaced arrangement. Basically, a grating comprising a plurality of elements may comprise a single set of the elements, wherein the elements are arranged in parallel, extending in a certain direction. However, it is also possible for a grating comprising a plurality of elements to comprise two sets of parallel elements, wherein the elements of the two sets extend in perpendicular directions. In any case, the size of the spaces between adjacent elements of a grating determines the size of items to be blocked by the grating.
Especially in case a grating is used in a marine environment, it may happen that the grating gets clogged as time passes, which hinders the flow of water through the opening in which the grating is present, and which may eventually result in total blockage of the opening. Such clogging is caused by a well-known phenomenon called biological fouling or biofouling.
In general, biofouling is the accumulation of microorganisms, plants, algae, small animals and the like on surfaces. According to some estimates, over 1,800 species comprising over 4,000 organisms are responsible for biofouling. Hence, biofouling is caused by a wide variety of organisms, and involves much more than an attachment of barnacles and seaweeds to surfaces. Biofouling is divided into micro biofouling which includes biofilm formation and bacterial adhesion, and macro biofouling which includes the attachment of larger organisms. Due to the distinct chemistry and biology that determine what prevents them from settling, organisms are also classified as being hard or soft. Hard biofouling organisms include calcareous organisms such as barnacles, encrusting bryozoans, mollusks, polychaetes and other tube worms, and zebra mussels. Soft biofouling organisms include non-calcareous organisms such as seaweed, hydroids, algae and biofilm “slime”. Together, these organisms form a biofouling community.
In several situations, biofouling creates substantial problems. Biofouling can cause machinery to stop working, water inlets to get clogged, and heat exchangers to suffer from reduced performance. Hence, the topic of anti-biofouling, i.e. the process of removing or preventing biofouling, is well-known. In industrial processes involving wetted surfaces, bio dispersants can be used to control biofouling. In less controlled environments, biofouling organisms are killed or repelled with coatings using biocides, thermal treatments or pulses of energy. Nontoxic mechanical strategies that prevent organisms from attaching to a surface include choosing a material or coating for causing the surface to be slippery, or creating nanoscale surface topologies similar to the skin of sharks and dolphins which only offer poor anchor points. As an alternative, ultraviolet light may be used for removing/preventing the formation of biofilm on wet surfaces. For example, WO 2014/014779 discloses a system for reducing biofouling of a surface of an optically transparent element subjected to a marine environment, including an LED for emitting ultraviolet radiation, a mount for directing emitted ultraviolet radiation toward the optically transparent element, and control circuitry for driving the LED.
U.S. Pat. No. 5,308,505 discloses an anti-biofouling system for use with a grating. During operation of the anti-biofouling system, the grating is illuminated with ultraviolet light for the purpose of keeping the grating holes and the grating surface free from biofouling.